In a long range LiDAR system short laser pulses are transmitted and directed towards a target surface according to a defined scan pattern using movable mirrors or refractive optics. In particular in an airborne LiDAR system the distance to the ground, i.e. the target surface, can be significant (up to 5 km) and the scanning rate of the scan pattern (typical 200-300 rad/s) quite high. This results in that the position of the moving optics, e.g. a sweeping mirror or a refractive scan unit, changes between when the light is directed towards the ground just after pulse transmission, and when it is redirected into the receiver optics when it returns after the reflection on the ground.
The round trip time for a pulse travelling at the speed of light (approx 300 000 km/s) at a ground distance of 5 km is 33 μs. If the scanner has a scan rate of 200 rad/s this results in a pointing difference of 6.6 mrad. This results in that the receiver will look 6.6 mrad away from where the laser beam hits the ground. A typical laser beam will have a beam size of typical 0.2-0.5 mrad. Thus to be able to collect the returning light, the field of view of the receiver has to be 10-20 times the size of the laser beam.
If the scanner can move the beam in complex patterns (2D) this displacement will occur on all sides of the laser beam, thus even double this field of view requirement on the receiver.
In addition to the angle deviation caused by the motion of the deflection element of the scanner, a further angle deviation between outgoing and incoming beam occurs along the flight path and is caused by the motion of the LiDAR carrier, e.g. an airplane, over the ground. However, this deviation is usually several magnitudes smaller and thus negligible for most applications.
Assuming again a round trip time of 33 μs, for a pulse travelling at the speed of light at a ground distance of 5 km, the deviation between transmitted and received beams along the flight path of an airplane travelling at a ground speed of 600 km/h is approximately 0.001 mrad, which is almost four orders of magnitude smaller than the deviation caused by the motion of the deflection unit of the scanner.
In terrestrial applications of long range LiDAR systems, e.g. in the field of construction monitoring or slope monitoring, typical measurement distances are shorter compared to the distances in common aerial applications. However, the scanning rate of the scan pattern might be higher, e.g. thanks to improved mechanical stabilization or because improved 3D models for the target surface are already available for fine tuning of the scan pattern. This fast scanning rate results in the same effect as described above for aerial applications, i.e. an angle difference between outgoing and incoming beam caused by the finite flight travel time of the beam and the fast movement of the deflection unit of the scanner between transmission and return.
For some special cases such as rectilinear scanning in one direction at a constant range, the angle difference between outgoing and incoming beam is compensated by statically displacing the receiver at the deflected focus. However, for scanning in two directions, e.g. using a circular scan pattern, and/or scanning at varying range this solution is technically not realizable or at least strongly limited.
The required large field of view of the receiver being 10-20 times the size of the laser beam has several drawbacks. For example, the solar background noise is strongly increased and limits the detection threshold for weak return pulse signals. Thus the transmission power needs to be increased for achieving a sufficient signal to noise ratio (S/N). Typically, the readout time of a detector depends on the size of the detector. Thus, the readout time of larger detectors is increased, i.e. limiting the overall scanning speed and/or the scanning resolution, and the detector bandwidth is typically reduced.